Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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086 FXUS63 KLOT 181730 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1230 PM CDT Wed Sep 18 2024 .KEY MESSAGES... - Today and tomorrow will be similar to the past few days with sunny skies, highs in the upper 80s, light winds, and daily lake breezes. - A few showers and storms may move into northwestern Illinois Thursday night. - Friday through the weekend will be noticeably more humid. Chances for widespread rain return this weekend. && .DISCUSSION... Issued at 327 AM CDT Wed Sep 18 2024 Through Tuesday: Patches of dense fog continue to meander through northern IL near the Fox and Kankakee River Valleys within a westward- traveling moisture axis. A Dense Fog Advisory is in effect until 800 AM for Lake (IL), McHenry, Kane, and DeKalb counties where webcams, ASOS/AWOS observations, and nighttime microphysics satellite imagery shows a solid "slug" of dense fog. Fog (in the Dense Fog Advisory and elsewhere) will erode quickly after sunrise. National infrared satellite imagery depicts an upper-level trough digging along the Pacific Coast upstream of an upper-level shortwave lifting into the northern Plains. Meanwhile, an upper-level low comprised of the remnants of two tropical systems continues to slowly meander through the Carolinas. Our area remains locked in a relative stagnant zone of the atmosphere between all features of interest. Just to illustrate the banality of our current pattern, the mean of all sampled winds from the surface to 200mb (basically the depth of the troposphere) from the past 9 RAOBS from ILX (7 AM Saturday through 7 PM Monday) is just shy of 12kt. This has been quite the slow pattern. With little change expected in the synoptic pattern over the next 24 to 48 hours, the weather today and tomorrow will be nearly the same as the past few days with sunny skies, highs in the mid to upper 80s, and daily lake breezes. The one departure today from the past few will be passing high clouds this evening and overnight, though they should clear in time for sunrise on Thursday. Thursday night into Friday, showers and storms should develop along and ahead a weakening cold front across parts of Iowa and Wisconsin. With the moisture axis feeding the convection lagging behind and supportive upper-level forcing departing further into Canada, the showers and storms may struggle to make much inroads into Illinois. Blended NBM guidance offers 30 to 50% chances for showers and storms before daybreak Friday focused along and west of I-39, which seems fair. After daybreak on Friday, low-level moisture characterized by surface dew points in the mid to upper 60s will finally slosh into our area as winds turn westerly. (Another gee whiz fact...The last time Chicago O`Hare logged a wind with a westerly component greater than 5 knots appears to have been on September 11. Again, that speaks to the persistence of the pattern this week). Anyway, the air will feel noticeably heavier by Friday afternoon, especially when combined with highs in the mid to upper 80s. While forecast soundings depict minimal capping by mid-afternoon, a dearth of large- scale forcing mechanisms suggests that development of any shower or storm Friday afternoon would be isolated at best, consistent with a 10 to 20% chance. This weekend, ensemble model guidance continues to favor a return of chances for welcome rainfall seemingly associated with an approaching upper-level shortwave emanating from the Plains. However, exactly how and when is not exactly discernible at this point given ensemble clustering exhibiting several forecast scenarios related to phasing opportunities with upper-level shortwaves parading along the jet stream near the US/Canadian border. At this point, blended NBM chances for thunderstorms ranging from 20 to 40% seems fair from Sunday onward keeping in mind they will change as time goes on. Borchardt && .AVIATION /18Z TAFS THROUGH 00Z FRIDAY/... Issued at 1229 PM CDT Wed Sep 18 2024 There are no aviation weather concerns through the 18Z TAF period. Near-5kt winds should favor a S to SE direction through mid- afternoon, but could vary at times. A shift to easterly is expected at the Chicago sites later this afternoon. Confidence is low on the timing of this shift, but best guess at the moment is around 20Z at both ORD and MDW. Light and variable winds can be expected tonight. Confidence is rather low on wind direction during the day tomorrow. It`s looking like we`ll establish a near-southerly or SSW direction when winds start to build a bit around mid-late morning. A shift to SE is then expected to reach MDW as early as mid-afternoon and looks to hold off until the evening to reach ORD. Speeds during the day should largely remain below 10 kt. Expect VFR throughout the period. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago